The present invention relates to a load sensor for measuring a load and converting the load into an electrical signal and a seat weight measuring apparatus employing the same.
Automobiles are equipped with seat belt devices and airbag devices as facilities for ensuring the safety of occupants. In order to improve the performance of the seat belt device and the airbag device, there are recent trends to control the actions of these safety devices depending on the weight (body weight) of an occupant. For example, the amount of gas for deploying the airbag and the energy absorbing (EA) load of the seat belt are adjusted depending on the weight of the occupant. For this purpose, some means are needed for measuring the weight of the occupant sitting on the seat. An example of such means is a proposal which involves arranging load sensors (load cell) at four comers of seat rails, obtaining vertical loads acting on the load cells, and summing them to determine the seat weight including the weight of the occupant.
A Load sensor, as mentioned above, is preferably a small-sized one capable of measuring about 50 kg at the maximum. Examples of such load sensors include a sensor comprising a sensor plate which is deflected depending on the received load and a strain gauge attached to (formed on) the sensor plate, and a sensor comprising a elastic member which is deflected depending on the received load and an electrical capacitance sensor detecting the displacement of the elastic member. A thick-film strain gauge may be used as the strain gauge.
As an example of the seat weight measuring apparatus and the load sensor as mentioned above, the devices disclosed in Japanese Patent Unexamined Publication No 2000-180255 (incorporated by reference) will be described. FIG. 9 is a side view schematically showing the entire structure of a seat weight measuring apparatus of the prior art. In this specification, the words “front and rear”, “left and right” without any definition mean the front and rear, the left and right for an occupant 1.
In this drawing, a seat 3, an occupant 1 on the seat, and seat weight measuring apparatuses 5 beneath the seat are shown. The seat 3 comprises a seat cushion 3a on which the occupant 1 sits, and a seat back 3b as a backrest. Seat adjustors 10 are disposed at four locations, front and rear on both sides of the bottom of the seat cushion 3a to project from the seat cushion 3a. It should be noted that only two of the adjustors 10 are shown which are located front and rear on the left side. The adjustors 10 on the right side are behind the adjustors 10 on the left side. The same is true for the following components of this apparatus in relation on the drawing. The seat adjustor 10 is an extending portion of a frame inside the seat 3 and is slidable in the front-and-rear direction on a seat rail 11 by adjustment operation of the occupant 1.
The seat rail 11 has a groove (not shown) and is a member extending in the longitudinal direction (front-and-rear direction) of a vehicle. The lower end portion of the seat adjustor 10 can slide within and along the groove. Two seat rails 11 are provided on the left and right sides beneath the seat cushion 3a, respectively. In a conventional seat without a seat weight measuring apparatus, the seat rails 11 are securely fixed to a seat bracket of a vehicle chassis by bolts. One of the seat rails 11 is provided, at one location near the rear end thereof, with an anchor fixing portion 12 for fixing a buckle 4 of a seat belt device 2. Tension of the seat belt 2 is exerted on the anchor fixing portion 12.
A pair of seat weight measuring apparatuses 5, i.e. the front and rear seat measuring apparatuses 5, are provided below the seat rail 11. The front and rear seat weight measuring apparatuses 5 are also provided below the right-side seat rail which is not shown. Consequently, the seat weight measuring apparatuses 5 are disposed at four locations, front and rear on both sides below the seat 3. Each seat weight measuring apparatus 5 comprises a seat supporting mechanism 17 and a displacement restriction mechanism 25 and disposed between the seat rail 11 and a seat fixing portion 19. In this example, the seat supporting mechanism 17 comprises a load sensor 13 and a deflection member 15 which are connected in series. The load sensor 13 detects the load applied to the seat supporting mechanism 17. The deflection member 15 is a member for enhancing the displacement (movement) of the seat rail 11 when the weight of the occupant is applied to the seat 3.
In this example, the displacement restriction mechanism 25 comprises a restriction bar 21 connected to the bottom of the seat rail 11, and a restriction block 23 formed on the seat fixing portion 19. The restriction bar 21 has an end portion 21 a of which diameter is enlarged just like a flange. The restriction block 23 has a recess 23a inside thereof. On the top end of the recess 23a, a flange 23b extending inwardly is formed. The restriction bar end portion 21a is fitted in the recess 23a of the restriction block with some spaces above and below, in front and rear, and on left and right thereof.
When an abnormal load is applied to the seat rail 11 so that the load sensor 13 and the deflection member 15 are deformed beyond a certain level, the restriction bar end portion 21a of the displacement restriction mechanism 25 collides with the inner wall of the recess 23a of the restriction block. For example, when the occupant 1 tending to move forward is restrained by the seat belt 2, tension is applied to the seat belt 2 due to the inertia of the occupant 1. At this point, the restriction bar 21 is pulled up to move upward. However, the movement of the restriction bar 21 is prevented by the lower surface of the flange 23b of the restriction block.
As mentioned above, the displacement restriction mechanism 25 for restricting the relative displacement between the seat and the seat fixing portion into a certain range is provided. In this case, when a force over a predetermined range (for example, a force exceeding the measurable range) is applied to the load sensor 13, the excess load is received by the displacement restriction mechanism (load limiting mechanism) 25, not the load sensor 13. Therefore, the required mechanical strength of the load sensor 13 can be significantly reduced, thereby achieving the miniaturization and reduction in cost of the load sensor 13.
Now, the relation between the displacement restriction mechanism 25 and the deflection member 15 of the seat supporting mechanism 17 will be described. If the deflection member 15 is not employed (a rigid member is employed) and the deformation of load sensor 13 over the measurable range is in the order of 0.1 mm, the space between the restriction bar end portion 21a of the displacement restriction mechanism 25 and the recess 23a of the restriction block should also be in the order of 0.1 mm, because the restriction bar end portion 21a is required to abut against the internal surface of the recess 23a of the restriction block as soon as the load exceeds the measurable range, so that the excess load is withstood by the displacement restriction mechanism 25.
That is, the displacement restriction mechanism 25 is required to have an operational precision in the order of 0.1 mm corresponding to the 0.1 mm stroke of the load sensor 13, which in turn requiring the parts dimensional precision and assembly precision in the order of 0.01 mm. This can not be fulfilled at all with current dimensional precision of the peripheral parts of the vehicle seat, which mainly consist of pressed products. In short, the small deflection stroke of the load sensor 13 calls for a high dimensional precision in the displacement restriction mechanism (load limiting mechanism) 25 and the peripheral members thereof. In this example, the deflection stroke of the seat supporting mechanism 17 in the measurable range or load bearing range of the load sensor 13 is amplified by the action of the deflection member 15 of the seat supporting mechanism 17. As a result, the dimensional precision and assembly precision requirements for members constituting the seat supporting mechanism 17 and the displacement restriction mechanism 25 can be alleviated.
Hereinafter, specific examples of the seat supporting mechanism and the displacement restriction mechanism will be described. FIGS. 10(A), 10(B) show the structure of a seat weight measuring apparatus of prior art, in which FIG. 10(A) is a general sectional side view and FIG. 10(B) is a plan view of a sensor plate. Shown in the uppermost portion of FIG. 10(A) is a seat rail 11. Under the seat rail 11, a sensor frame upper plate 31 and a sensor frame 33 are fixed by means of bolts 32. The sensor frame upper plate 31 is a durable plate having a hole 31a at the center. The sensor frame 33 has a saucer-like configuration with a recessed central portion. Formed around the upper external periphery of the frame 33 is a flange 33a which is fixed to the sensor frame upper plate 31 by means of the bolts 32, as described above. The bottom plate 33b of the sensor frame 33 is provided with a hole 33c formed at the center thereof.
A sensor plate 37 is fixed by means of bolts 35 to the lower surface of the sensor frame upper plate 31. The sensor plate 37 is made of a stainless steel and is a rectangular plate with a thickness of 3 mm, a width of 20 mm, and a length of 80 mm. As shown in FIG. 10(B), the sensor plate 37 is provided with a central through hole 37c formed in the central portion and with bolt holes 37a formed in the both side portions. Attached to the upper surface of the sensor plate 37 are resistor-type strain gauges 37b, a pair of them being attached on each front and rear portions of the plate (left and right portions in FIG. 10(B)). These resistor-type strain gauges 37b are for measuring the load acting on the sensor plate 37, by detecting the distortion of the plate 37.
Fitted into the hole 37c located at the center of the sensor plate 37 is a central shaft 39, and the sensor plate 37 and the central shaft 39 are fixed to each other by means of a nut 39a. Into the holes 37a located at both sides of sensor plate 37, bolts 35 are inserted upwardly, thereby fixing the sensor plate 37 to the sensor frame upper plate 31.
The central shaft 39 is a cylindrical shaft having several steps and flanges. The central shaft 39 comprises, from its upper side, an upper nut 39a, a flange 39b, a sensor frame penetrating portion 39c, a small diameter portion 39d, a lower nut 39e, and the like. The upper nut 39a fixes the sensor plate 37 as described above. The nut 39a enters into the central hole 31a of the sensor frame upper plate 31. In the nominal state, the gaps between the nut 39a and the hole 31a are, for example, 0.25 mm in the vertical direction and 0.5 mm in the radial direction. When the seat rail 11 is subjected to a large force and the deformation of parts including the sensor plate 37 is increased, the nut 39a comes in contact with the internal surface of the hole 31a. At this point, the further deformation of the sensor plate 37 is stopped. That is, the nut 39a on the central shaft and the central hole 31a of the sensor frame upper plate compose the displacement restriction mechanism.
The outer diameter of the flange 39b of the central shaft 39 is greater than the diameter of the central hole 33c of the sensor frame 33, the lower surface of the flange 39b facing the upper surface of the sensor frame bottom plate 33b with a gap of 0.25 mm in the nominal state. When the seat rail 11 is subjected to a force acting upward and the deformation of the sensor plate 37 advances, the sensor frame 33 is lifted and the central upper surface 33d of the frame bottom plate 33b comes in contact with the bottom surface of the central shaft flange 39b. Meanwhile, a gap of 0.7 mm exists between the outer periphery of the sensor frame penetrating portion 39c of the central shaft 39 and the inner periphery of the sensor frame central hole 33c in the nominal state. This portion also composes the displacement restriction mechanism.
The small diameter portion 39d of the central shaft 39 extends downward from the sensor frame penetrating portion 39c with decreasing its diameter stepwise. The nut 39e is screwed to the end of the small diameter portion 39d. Fitted onto the outer periphery of the small diameter portion 39d are, from its upper side, a washer 41, a rubber washer 43, a sensor base 45, another rubber washer 43, and another washer 41. The washers 41 are made of metal. The rubber washers 43 expand and contract by about 0.5 mm in the sum of two sheets, upper and lower, for a load variation of about 50 kgf in the vertical direction. The rubber washers 43 serve to absorb dimensional difference and distortion between the seat rail 11 and the seat fixing portion (a seat bracket 47).
The sensor base 45 is a metal plate and comprises a lowermost member of the seat weight measuring apparatus. The upper and lower washers 41, the upper and lower rubber washers 43, and the sensor base 45 are retained between the lower step of the sensor frame penetrating portion 39c of the central shaft 39 and the lower nut 39e. The end 45b of the sensor base 45 is fixed to the seat bracket 47 by means of a bolt which is not shown. The seat bracket 47 projects from the vehicle chassis.
The general action of the seat weight measuring apparatus of FIGS. 10(A), 10(B) will be summarized. The weight of a seat and a occupant loaded on the seat rail 11 are normally transmitted via the sensor plate 37 to the central shaft 39, the rubber washers 43, the sensor base 45, and the seat bracket 47. At this time, the sensor plate 37 gives rise to a deflection roughly proportional to the load which is detected by the strain gauges 37b, to measure the load acting on the sensor plate 37 in the vertical direction. The weight of the occupant is obtained by summing the loads measured by the respective load sensors, front and rear on both sides, and subtracting the known weights of the seat and the seat rail from the sum.
On the other hand, when an abnormal force exceeding the measurable range or load limit of the load sensor is applied to the seat rail 11, the central shaft nut 39a comes in contact with the internal surface of the central hole 31a of the sensor frame upper plate, or otherwise, the central shaft flange 39b or the sensor frame penetrating portion 39c comes in contact with the sensor frame bottom plate 33b. This action of the displacement restriction mechanism prevents excessive deformation of the sensor plate 37 while securely connecting the seat rail 11 and the seat bracket 47.
For eliminating the influence of noise under the on-vehicle circumstances, generally the electrical output should be increased, so it is required to apply deformation distortion as large as possible to the sensor plate. For this, portions under the resistors for detecting deformation of the sensor plate are required to be deformed to obtain the maximum distortion within a range allowed by the base and the laminated layers (sensor). When the distortion is locally concentrated, however, the sensitivity is unsteady according to the distortion pattern of the sensor plate and further localized concentration occurs due to impact or the like, the strain on a portion may exceed the allowable limit and the portion may be broken. Therefore, in order to effectively utilize the allowable range for the sensor plate and the laminated layers, the sensor plate should be made in such a manner as to disperse the deformation stress and to unify the distortion on the surface to obtain distortion corresponding to 70% or more of the maximum distortion about the resistors on the sensor plate.
Hereinafter, the structure of the sensor plate and its peripheral parts will be described. FIGS. 11(A)–11(C) are illustrations showing a structure example of a sensor plate of the conventional seat weight measuring apparatus. FIG. 11(A) is a plan view of the sensor plate, FIG. 11(B) shows a section taken along a line X–X′ of FIG. 11(A), and FIG. 11(C) is a circuit diagram of the sensor. In FIGS. 11(A)–11(C), the sensor plate is marked with a numeral 51.
A sensor 50 comprises a sensor plate (spring member) 51 as a base thereof and an insulating layer (lower insulating layer) 52 formed on the sensor plate 51 for electrical insulation. Selectively formed on the insulating layer 52 is a wiring layer 53. Further selectively formed on the wiring layer 53 is a resistor layer 54 to compose a strain gauge. In addition, an insulating layer (upper insulating layer) 55 is applied over these layers as a protective layer. In this manner, the electrical circuit including resistors is directly laminated on the spring member 51, thereby reducing the working cost and the assembly cost and further improving the heat resistance and the corrosion resistance.
The sensor plate 51 is a rectangular plate having two necks 52c as a whole. The sensor plate 51 is provided with a central hole 51a in the center thereof and bolt holes 51b formed in both end portions thereof. The sensor 50 is formed around the central hole 51a and between the central hole 51a and the bolt holes 51b. V-shaped concaved necks are provided in opposite side edges of regions 51c between the central hole 51a and the bolt holes 51b. These necks ensure positions to be deformed of the sensor plate 51, thereby preventing positional variation of distortion and stabilizing the sensitivity of the sensor 50.
The sensor 50 is substantially symmetrical about the center of the central hole 51a. That is, the sensor 50 is composed of four resistor-type strain gauges of which two 54a, 54b to be applied with tensile distortion are arranged near the bolt holes 51b (near the ends), and the other two 54c, 54d to be applied with compressive distortion are arranged near the central hole 51a (central side). The four resistor-type strain gauges 54a, 54b, 54c, and 54d are connected to each other by wirings 53a, 53b, 53c, and 53d to form a bridge circuit shown in FIG. 11(C). Squares marked with numerals 1, 2, 3, 4 in FIG. 11(C), are terminals.
Arranged between the resistor-type strain gauges 54a, 54c and the resistor-type strain gauges 54b, 54d is a sensitivity control resistor 54e. It should be noted that the load may be obtained by conversion from deflection of the sensor plate 51 detected by electrical capacitance pressure sensors or Hall elements, instead of the detection of distortion of the sensor plate 51 being detected by the resistor-type strain gauges 54a, 54b, 54c, and 54d. 
In the load sensor, for example as shown in FIG. 11(A), the load is measured by causing deformation of the sensor plate 51 due to the load, detecting the distortion due to the deformation by the strain gauges 54a, 54b, 54c, and 55d shown in FIG. 11(C), and detecting the output of the bridge circuit composed of these strain gauges as described above.
However, variation in value of resistance of the strain gauge depends on not only the distortion but also the temperature. That is, when the environmental temperature of the seat weight measuring apparatus rapidly changes, the temperature of the sensor base 45 having a relatively large surface area is changed. The change in temperature is transmitted to the sensor plates 37 (51) via the bolts (sensor post) 49. A temperature difference is generated among the strain gauges 54a, 54b, 54c, and 54d on the sensor plate 51. This temperature differences produces a difference in values of resistances. Accordingly, the output of the bridge circuit composed of these strain gauges is outputted, causing error in weight measurement.